(1) Field of the Invention
The present invention relates to a sheet delivering and feeding device for use in a copier, printer, plain paper facsimile, etc., in particular relating to a sheet delivering and feeding device which delivers sheets as recording medium, one by one, to the image forming station, providing a prescribed sheet distance between one and the next.
(2) Description of the Prior Art
Copiers, printers, plain paper facsimiles and other image forming apparatuses have a sheet delivering and feeding device, that is, a device for delivering sheets, one by one, to the image forming station, providing a prescribed sheet distance between one and the next sheet.
For example, an image forming apparatus shown in FIG. 2, has a sheet delivering and feeding device 101 in which sheets 105 stacked on a sheet tray 108 in the sheet delivering and feeding portion 101 are delivered one by one from the topmost sheet by the function of a feed roller 112. Feed roller 112 has a frictional member 113 pressed thereagainst in order to stop double delivery of sheets. That is, when two sheets are fed therebetween, only the top sheet is delivered while the delivery of the lower sheet is stopped by frictional member 113, thus achieving sheet-by-sheet feeding.
The sheet thus delivered is detected by a sheet feed sensor 115 as a sheet feed detecting means, sensing its front end, prior to the delivery to the image forming station, in particular, the transfer position. In time with the detection of the front end of the sheet, the operation of forming an image on a photoreceptor drum 121 is started. In this way, the time when the front end of the sheet reaches the transfer position, more specifically, the contact nip between a transfer roller 122 and photoreceptor drum 121, is synchronized with the front end of the image formed on the surface of photoreceptor drum 121.
Rotation of feed roller 112 is stopped at a time (e.g., after a certain elapse of time based on a timer) after the front end of the sheet passed by sheet feed sensor 115 and has been held between photoreceptor drum 121 and transfer roller 122. Then, after the rear of the sheet passes by sheet feed sensor 115, a sheet-pickup signal for triggering the drive to feed roller 112 for delivering the next sheet to the image forming station is produced. In this way, the sheets will be successively delivered sheet by sheet with a prescribed sheet distance in between, avoiding overlap between the rear of the delivered sheet and the front of the subsequent sheet.
In accordance with the above conventional art shown in part by FIG. 3, it is possible to perform successive sheet feeding by avoiding overlap between the rear of the preceding sheet and the front of the next sheet, but the interval between the two successive sheets delivered is constrained, even in the optimal case, by the distance between the sheet feed sensor and the feed roller unit. Therefore, it is impossible to further shorten the sheet interval. As a result, it is impossible to enhance the print speed or shorten the time from feeding a sheet to print-out, without increasing the rotational speed of photoreceptor 121.
Repeatedly, when the speed of forming an image on the photoreceptor cannot be increased, it is impossible to further enhance the print speed if the interval between sheets is determined by the distance between the sheet feed sensor and the feed roller as stated above. In particular, in the configuration where a sheet feed sensor detects the front end of the sheet and this detection is used for generating a timing signal for starting the operation of forming an image on the photoreceptor, if feeding of the next sheet is performed in response to this timing signal, it is impossible to shorten the sheet interval by any means and hence it is practically unfeasible to enhance the print speed by shortening the sheet interval.
In the paper delivering and feeding device shown in FIG. 2, after the completion of the feeding operation of a single sheet by means of feed roller 112, the operation of feed roller 112 is stopped but, it is passively rotated by the sheet being conveyed. Accordingly, when the rear of the sheet passes by feed roller 112, the front end of the next sheet may enter and pass by the position of frictional member 113 in conformity with the rotation of feed roller 112 so that it may be set into a stand-by state where the front end is drawn past (in the dragged state). In this state, the delivery of the next sheet, will be started when feed roller 112 starts to be rotated after the rear of the preceding sheet is detected by sheet feed sensor 115 as described above. Therefore, the sheet interval may be inconsistent depending upon the delivered state of the next sheet, and will optimally be the distance between sheet feed sensor 115 and feed roller 112.
As stated above, in the conventional art, since the start of feeding the next sheet is controlled in response to the detection of the rear of the preceding sheet by sheet feed sensor 115, the interval between sheets will be definitely determined by the distance between the feed roller and the sheet feed sensor. The only way to further shorten the sheet interval, would depend on expectation that the next sheet may be delivered simultaneously with the passage of the rear of the preceding sheet through the sensor. However, this cannot be realized and therefore cannot be a factor which provides stably consistent shortening of the sheet interval.